Measurements of acoustic responses of gaseous propellant injectors

Abstract

In a stability analysis of a gaseous propellant rocket combustor the influence of the injector design upon the combustor dynamics must be known quantitatively. The combustor injector interaction is generally specified by the injector response factor which describes the manner in which the propellants' burning rate responds to a given pressure oscillation within the combustion chamber. This paper is concerned with the experimental determination of the response factors of a variety of coaxial gaseous propellant rocket injectors. The injector response factors are obtained from injector admittance data measured under cold-flow conditions simulating those observed in rocket motors experiencing axial instability. The experimental data are found to be in reasonable agreement with theoretical predictions obtained by using the Feiler and Heidmann analytical model. Also presented in this paper are data that describe the dependence of the injector response factors upon the open-area ratio of the injector configuration and the length of the injector orifices. These data indicate that although there is no observable change in the magnitude of the response factor at resonance, an increase in the orifice length decreases the injector resonant frequency and the band-width of the injector response curve. Furthermore, it is noted that for a given mass flux through the injector orifices, a change in the injector open-area ratio has a considerable effect on the characteristics of the injector response factor. The relevance of these data to stability considerations of rocket motor combustors and gaseous fuel fired burners is indicated.